Wireless communication devices use receivers to convert received radio frequency (RF) energy into baseband signals, centered at zero frequency or “DC”. Information carried in the baseband signals is then recovered using any of a variety of signal processing steps depending on the particular techniques used to modulate the information onto the baseband signal prior to its wireless transmission.
RF device designers are continually seeking ways to improve the performance of receivers in order to accurately recover the information carried in a received RF signal. For example, one type of receiver architecture, known as a “superheterodyne” receiver, downconverts received RF signals to baseband in two stages: first from RF to an intermediate frequency (IF), and then from IF to baseband. A superheterodyne receiver enjoys good rejection of interference but it is relatively complex to design, uses circuitry that occupies a substantial amount of area in an integrated circuit and consequently has a relatively high cost.
Another type of receiver architecture is one that directly downconverts received RF signals to baseband. This type of receiver is referred to as a “direct conversion” receiver. An advantage of a direct conversion receiver is that it is simpler to design and implement and occupies less area in an integrated circuit. A disadvantage of a direct conversion receiver is that it experiences interference called self-mixing interference or second order inter-modulation interference, also known as “IM2” interference. The spectrum associated with IM2 interference is caused by the spectrum of an interfering signal being captured and down-mixed, together with the spectrum of the desired signal, by the receiver. The spectrum of the IM2 interference is centered at DC and thus overlaps with the desired baseband signal that is also centered at DC. When the level of the IM2 interference is significant, the performance of the receiver is severely degraded, making it more difficult to recover information in the baseband signal.